Cold cathode vacuum tube and circuit



Feb. 27, 1962 B. e. FIRTH 3,023,364

COLD CATHODE VACUUM TUBE AND CIRCUIT Filed May 29. 1959 2 Sheets-Sheet 1 W Ml HI m 1 2/ l I m I I I l i l l m ll 20 J 152:: c a l /7 a L Q u 22 I 1 H INVENTOR 1 BQEE/VARB 6". fiE'T/l A I 6 1M 711m Mi u ATTORNEYS Feb. 27, 1962 B. G. FIRTH 3,023,364

COLD CATHODE VACUUM TUBE AND CIRCUIT Filed May 29, 1959 2 Sheets-Sheet 2 DGQOOQOUQBOOOCO INVENTOR 35mm)? 6'. F/ew/ hid/m4 cl flaw ATTORNEYS United States Patent 3,023,364 COLD CATHODE VACUUM TUBE AND CIRCUIT Bernard G. Firth, Newark, N.J., assignor to Tung-Sol Electric Inc., a corporation of Delaware Filed May 29, 1959, Ser. No. 816,800 9 Claims. (Cl. 328-252) This invention relates to a cold cathode vacuum tube with associated circuitry designed to keep the cold cathode in an electron emitting condition and to provide adequate external control of the anode-cathode current. It has particular reference to an arrangement of control electrodes within the tube envelope to insure eflicient operation.

Prior art tubes containing sustaining electrodes have been difiicult to control for several reasons. First, the electron emission from the cold cathode produces electrons which vary widely in speed. This wide range of velocity produces a characteristic which has no sharp cut-01f as the control grid is progressively made more negative with respect to the cathode. Second, with a single control electrode with closely spaced grid wires p0 sitioned between the sustaining grid and the cathode, there is a tendency to cut-ofi electron emission when the control electrode is made negative. Placing a single control electrode between the sustaining electrode and the anode reduces the amplification factor of the device and makes it less efficient. The present invention uses two control grids, both having grid wires which are widely spaced in comparison to the spacing of the sustaining grid, one of said grids being positioned between the cathode and the sustaining grid while the other is between the sustaining grid and the anode. This arrangement produces a sharper cut-off and permits an adequate control of the anode current without increasing the internal anode resistance.

One of the objects of this invention is to provide an improved cold cathode vacuum tube discharge device which avoids one or more of the disadvantages and limitations of prior art arrangements.

Another object of the invention is to provide an efficient system of control grids for a cold cathode high vacuum tube.

Another object of the invention is to increase the sharpness of cut-off of the anode current characteristic.

.Another object of the invention is to provide two control grids, one for controlling the slow electrons, and one for controlling the fast electrons.

Another object of the invention is to insure sustained electron emission from a cold cathode in an electron discharge device.

The invention comprised an evacuated envelope containing a cold cathode and an anode. A sustaining grid electrode is positioned between the anode and cathode for sustaining electron emission from the cold cathode at all times when the discharge device is operating. A first control electrode is positioned between the sustaining electrode and the anode and a second control grid electrode is positioned between the sustaining electrode and the cold cathode.

A feature of the invention includes a st lstaining electrode having closely spaced grid wires while the two control electrodes have grid wires with spacings substantially greater than the sustaining grid.

A second feature of the invention includes circuit means foriconnecting the sustaining grid electrode to the cathode in series with a ballast resistor and a source of potential. The two control grids are connected together and are coupled to an input terminal for receiving an input signal.

For a better understanding of the present invention, to-

3,023,364 Patented Feb. 27, 1962 gether with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings.

FIG. 1 is a cross sectional view of the cold cathode vacuum tube showing the arrangement of the three grid electrodes, the anode, and the cathode.

FIG. 2 is a side view of the internal components ofthe vacuum tube of FIG. 1 taken partly in cross section.

FIG. 3 is a greatly enlarged cross sectional view of a portion of the cathode-anode space showing the approximate position of the three grid electrodes and the spacing of the grid wires.

FIG. 4 is a schematic diagram of electrical connections showing how the vacuum tube is connected in an operating circuit.

Referring now to FIGS. 1, 2, and 3 the discharge device includes an evacuated envelope 10 which may be made of glass. Within the envelope is positioned a cold cathode 11 which is generally formed with a metallic base such as a nickel sleeve, vw'th a porous, sponge-like coating of magnesium oxide on its outside surface. The cold cathode is generally hollow and encloses a filament 12 which is employed to heat the cathode only during the processing operation. The discharge device also includes an anode 13, spaced apart from the cathode and connected to a lead-in wire for connection to an external circuit. Between the anode and cathode are mounted three grids, one grid 14 is a-sustaining grid and is employed only for the purpose of sustaining the electron emission from the cold cathode. Between the cathode and the sustaining grid is a control grid 15 and between the sustaining grid and the anode is another control grid 16. It should be noted that the sustaining grid 14 includes closely spaced wires (see FIG. 3). The control grids 15 and 16 have grid wires with considerably more spacing than that of the sustaining grid wires. The spacing between the grid wires of the outer control grid 16 is about twice that of the wires of the inner control grid 15 and about four times that of the wires of the sustaining grid. This arrangement of grid wire spacing is an important feature of the invention. The wide spacing of the wires of grid 15 insures the field set up by the positively charged sustaining grid will be eifective, even during negative potential swings of the control grids, to prevent cut-off of the tube. The still wider spacing of the wires of the outer control grid 16 insures that'the field set up by the positively charged anode will be effective to attract electrons through the grid 16 even during positive potential swings of the grids.

FIGS. 1, 2, and 4 show an additional filament 17 which is connected between supporting conductors 18. This filament when rendered incandescent starts the emission of electrons from the cold cathode 11 when the tube is put into operation. Filament 17 is raised to an electron emitting temperature by external means and the electrons given off from the filament are projected through a hole 20 in the anode and are drawn to the cathode where they react with the magnesium oxide coating thereon to start emission. While the filament 17 is a practical and useful means for starting emission in the cold cathode vacuum tube it is not a part of this invention as it has been described and claimed in the copending application Serial No. 802,536 filed March 2, 1959 of the present applicant.

The components listed above are generally positioned in the tube between two insulating spacers 21 and 22 which may be of mica. The grids 14, 15, and 16 may be supported by larger conducting supports, as shown, or they may be supported in any other way necessary to retain them in operating position.

A suitable operating circuit for the tube described above is shown in- FIG. 4 and includes circuit means 23 for connecting grids and 16 together. These grids are coupled to an input terminal 24 by any type of coupling circuit such as a transformer or the capacitor-resistor network shown in the drawing. Capacitor 25 of the couplirig circuit is connected in series between input terminal 24 and the two grids while resistor 26 is connected to an adjustable contact 27 on a voltage divider which includes a resistor 30 connected across a source of potential 32. The negative terminal of the potential source is connected to a second input terminal 33 which may be grounded.

The load or output circuit is connected to two output terminals 36, 37, these terminals being coupled by a transformer 38 or any other convenient coupling means to the anode-cathode circuit of the tube. This circuit includes the potential source 32, a portion of the coupling means, which in this case is a primary winding 40 of transformer 38, a disconnect switch 41, and the anode 13.

The sustaining grid electrode 14 is connected to the positive terminal of the source of potential 32 in series with an adjustable resistor 42. One end of the starter filament 17 is connected to the cathode 12 and the other end of the filament is connected in series with a small resistor 4-3, a switch 44, and a small source of potential 45 the negative terminal of which is connected to the cathode. Switch 44 and switch 41 are coupled together so that on starting, when switch 44 is closed, anode switch 41 is open.

When the tube is put into operation switch 44 is closed and switch 41 is open. This action sends current through starter filament 17 and projects a stream of electrons through hole in anode 13 (FIGS. 1 and 2) and starts the electron emission from the cathode surface. This emission is maintained by the sustaining grid electrode 14 which maintains an electric field at all times between the cathode 11 and the grid mesh 14 due to the application of the source of potential 32. Switch 44 is now opened and switch 41 is closed, thereby cutting off current to the starter filament and connecting the anode 13 through winding 40 to the source of potential 32. The tube will now function as an amplifier until the sustaining voltage on grid 14 is removed.

Although the tube will usually start without momentary opening of the anode circuit, as by switch 41, much more effective starting is achieved when the anode circuit is so opened. It is believed that the momentary removal of potential on the anode permits electrons emitted from the cathode that miss the grid wire to return to the cathode to drive out more electrons. Also, opening of this anode circuit during energization of the starter filament cuts out noise momentarily generated by the starter filament. A positive potential on the control grids, negative with respect to the sustaining grid, does not interfere with the starting.

The foregoing disclosure and drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense. The only limitations are to be determined from the scope of the appended claims.

I claim:

1. An electron discharge device comprising; an evacuated envelope containing a cold cathode and an anode, a sustaining grid electrode positioned between the anode and cathode for sustaining electron emission from the cold cathode, a first control gird electrode positioned between the sustaining electored and the anode, a second control grid electrode positioned between the sustaining electrode and the cold cathode and connected to the first control grid electrode, and lead-in conductors sealed in said envelope for connecting said electrodes to an external circuit.

2. An electron discharge device comprising; an evacuated envelope containing a cold cathode and an anode, said cold cathode comprising a metal base covered with 4 magnesium oxide, a sustaining grid electrode positioned between the anode and cathode for sustaining electron emission from the cold cathode, a first control grid electrode positioned between the sustaining electrode and the anode, and a second control grid electrode positioned between the sustaining electrode and the cold cathode.

3. An electron discharge device comprising; an evacuated envelope containing a cold cathode and an anode, a sustaining grid electrode positioned between the anode and cathode for sustaining electron emission from the cold cathode, a first control grid electrode positioned between the sustaining electrode and the anode, and a second control grid electrode positioned between the sustaining electrode and the cold cathode and connected to the first control grid electrode, all of said grid electrodes formed with spaced parallel conductors, the sustaining electrode having a spacing between conductors substantially less than the spacing between conductors of the first and second control electrodes.

4. An electron discharge device as set forth in claim 3 wherein said first control electrode has a spacing between conductors which is substantially greater than the spacing between conductors of the second control electrode.

5. An electron discharge device comprising; an evacuated envelope containing a cold cathode and an anode, said cold cathode comprising a metal base covered with magnesium oxide, a sustaining grid electrode positioned between the anode and cathode for sustaining electron emission from the cold cathode, a first control grid electrode positioned between the sustaining electrode and the anode, a second control grid electrode positioned between the sustaining electrode and the cold cathode and connected to the first control grid electrode, all of said grid electrodes formed with spaced parallel conductors, the sustaining electrode having a spacing between conductors substantially less than the spacing of the first and second control electrodes, and lead-in conductors sealed in said envelope for connecting said electrodes to an external circuit.

6. An electron discharge device comprising; an evacuated envelope containing a cold cathode and an anode, a sustaining grid electrode positioned between the anode and cathode for sustaining electron emission from the cold cathode, a first control grid electrode positioned between the sustaining electrode and the anode, a second control grid electrode positioned between the sustaining electrode and the cold cathode, lead-in wires sealed in said envelope for connecting said electrodes to an external circuit, circuit means for connecting said sustaining electrode to a source of positive potential, and circuit means for coupling said first and second control electrodes to an input terminal.

7. An electron discharge device as set forth in claim 6- wherein said anode and cathode are connected in series with a source of potential and a load.

8. An electron discharge device according to claim 7 including a filament within the envelope for initiating electron emission from the cathode, an energizing circuit for said filament and switch means for simultaneously opening the connection between said source and said anode and closing said energizing circuit for said filament.

9. An electron discharge device as set forth in claim 6 wherein said sustaining electrode is connected to the cathode in series with a ballast resistor and a source of potential.

References Cited in the file of this patent UNITED STATES PATENTS 2,607,021 Von Gugelberg Aug. 12, 1952 2,802,127 Dobischek et al. Aug. 6, 1957 2,842,706 Dobischek et al. July 8, 1958 

